Advice on RC8 star collimation

[mldee (Mike)]

Finally, clear nights and temps still above zero!
Put the OCAL-collimated RC8 with imaging train on the AM5 with a 183mm binned 4 screwed onto the OAG housing.

First pics before any adjustments looked promising, although some donut variation across the image.

I have two questions:

1 - What is causing the variations in roundness
and
2- Which screws to tweak to improve roundness: Primary, secondary or image train adjusters?

Pic shows setup and initial image before any adjustments.
Thanks,

[MarkInSpace (Mark)]

Hi Mike,
My approach may be naive, but it seems to work for me:
I approach RC collimation as 2 tasks - out of focus (OOF) roundness, and
centering the central OOF point.

I first Center the secondary using my eyes.
The the OOF roundness is adjusted with the collimation screws on the body of the telescope. Use a circle in your display to help get roundness correct.
Next adjust the secondary to Center in the image. When it gets close, you will see most of the OOF images are centered.
Here’s the type of resolution I am getting with my RC-8 carbon.
https://www.iceinspace.com.au/forum/...e.php?a=315489